十万个为什么·科学启蒙30篇（3）

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为什么切洋葱会流泪

Cutting an onion damages its cells and releases enzymes that convert sulfur compounds into volatile gases.
One gas, syn-propanethial-S-oxide, rises into the air and dissolves in the moisture on your eyes’ surface.
This forms mild sulfuric acid, which irritates the cornea and triggers tear glands to flush it out.
Tears contain lysozyme—an enzyme that kills bacteria—so crying also protects your eyes from infection.
Chilling onions before cutting slows enzyme activity and reduces gas release significantly.
Using a sharp knife causes less cell damage than a dull one, lowering the amount of irritant produced.
Wearing swim goggles or cutting near a running fan helps divert the gas away from your face.
Onions bred for low sulfur content—like sweet Vidalias—produce far less of the tear-inducing compound.
Garlic and leeks cause similar reactions, but their gases are less airborne and more pungent-smelling.
Interestingly, some people lack the gene variant that makes them sensitive to this particular onion compound.